When you say she drags her legs, is she pulling with just the front legs with the rear legs flat on the ground or is she dragging her toes?
Stiffness and toe dragging are typical signs of pain when moving the leg forward. It is common with older dogs that have arthritis particularly of the hips. There are also neurologic problems such as degenerative myelopathy and spinal arthritis that can result in dragging toes that progresses to collapsing and dragging the entire leg.
You can give glucosamine. It may help some, but rarely helps enough to be the only treatment. Antiinflammatory drugs do a much better job of reducing inflammation of arthritic joints and reducing pain. There are a number of non steroidal antiinflammatory drugs labeled for use in dogs. They are significantly more effective at controlling pain and are safer than OTC human drugs. Examples include RIMADYL/NOVOX (carprofen), PREVICOX (firocoxib), DERAMAXX (deracoxib), ETOGESIC (etodolac), METACAM (meloxicam), ZUBRIN (tepoxalin).
The first step is to have your vet check your girl to get a diagnosis. I can make guesses based on what you type, but I can not diagnose accurately without a hands on physical exam and the ability to do diagnostic tests if indicated.
Let me know if you have follow up questions.
Collapsing makes me think there is a neurologic component. Toe dragging can be either pain (hip arthritis) or neurologic. Most 12 year old Labs have some arthritis and reducing pain with NSAIDs helps sort out what is pain and what is something else.
Degenerative myelopathy is a progressive degeneration that generally starts with toe dragging and hind leg dragging. There is no treatment, but again treating concurrent pain often improves the gait and quality of life.
Other spinal disease is possible including disc disease and arthritis.. Some of these respond well to corticosteroids (prednisone), but these drugs and NSAIDs should not be used together. The risk of GI ulcers is increased. X-rays can help pick up spinal changes, but may miss disc protrusion.
If you are able to take your girl to her vet to get a hands on diagnosis, you are more likely to get an effective treatment. I do not think glucosamine and/or condroitin will give her as much benefit as other drugs. Glucosamine can be used with other drugs and may provide some additional improvement particularly for arthritis.